The present invention relates to a propulsion system for ships, which propulsion system comprises one or several impellers mounted on one shaft each, which impeller/s establishes/establish a force that drives the ship forward. The impeller, being rotatable in an impeller house by means of the driving shaft, is provided with blades of the propeller type, which produce the jet stream backwards.
The propulsion of ships, preferably fast moving ships, both military and civilian ones, through water jet arrangement, comprising impellers are generally known. The housing surrounding the rotating impeller provided with blades is fixedly mounted to the rear portion of the hull. The impeller is typically driven by a steel shaft extending towards the stem by suitable arrangements which in turn are driven by one or several engines within the hull. A tube-like water inlet, which slopes somewhat downwards in the moving direction, is provided in front of the impeller housing in order to supply a large amount of water. The driving shaft thus runs through said tubular water inlet. The ship is controlled by means of steering devices downstream the impeller housing (or housings), which may direct the jet stream in different directions. The jet stream may also be directed forwards to give a decelerating effect.
As the driving shaft of the impeller extends through the water inlet, the incoming flow of water to the impeller is disturbed to some extent, which implies that an unevenly distributed load on the blades of the impeller is created. Said uneven load implies that a bending moment is transferred to the impeller inwards towards the attachment point of the impeller. Because of these varying forces influencing the impeller and its attachment point, very high requirements are put on the arrangement of the bearings and sealings. It is known from SE 424 845 to solve said problem by arranging the impeller fixedly mounted to the shaft and to arrange a bearing arrangement allowing a certain angle deviation. However, said solution is relatively heavy, especially since it requires a design with a bending rigid driving shaft (in order not to risk too great angle deviations), which shaft thus is very heavy. It is not unusual that only the weight of the driving shaft in such a design amounts to about 10% of the total weight of the water jet device (including the weight of the of the pump unit including stator part with guide vanes, thrust and journal bearing arrangement, impeller and impeller housing and the steering and reversing gear).
Another known solution is shown in SE 457 165 and SE 504 604, wherein a bearing arrangement is used which cannot handle angle deviations and wherein a flexible coupling between the driving shaft and the impeller is used instead, the flexible coupling being intended to handle the angle deviations. Also said last mentioned solution leads to a heavy design, especially since the coupling as such implies an additional weight. Further, it implies a drawback as the coupling is provided at a critical position as to flow, which makes it difficult to obtain optimal flow conditions. Moreover, the coupling implies a power limitation. It is realized that a detail which limits the power transmission is not desirable in such applications, as, especially with such applications, it many times is desirable to be able to transfer a lot of power, often in the interval of 3-30 MW. The design according to SE 504 604 instead shows the use of a flexible coupling and is directed to an embodiment, which makes it possible to dismount the bearing unit backwards. This implies i.a. that the guide vanes, which transmit the force from the impeller to the stator shell, must have a very limited extension. For long it has been a desire to reduce the weight in order to increase the power density (with power density is meant the maximal power output divided with the weight of the water jet unit, comprising the weight of the pump unit including stator part with guide vanes, thrust and journal bearing arrangement, impeller and impeller housing and the steering and reversing gear). With known designs it is probably difficult to achieve a power density above 1 kW/kg for a water jet having an inlet diameter above 1 m, which is an undesired and serious limitation. As is evident for the skilled man the power density for the same kind of design does decrease with increased size.
An objective of the invention is to find an optimal solution of the above described complex of problems. Said objective is achieved by a driving system for propulsion of a ship comprising an impeller, a stator shell, and an impeller housing for the achievement of a water jet, a shaft for driving the impeller and a bearing arrangement for the shaft in the stator shell, wherein said bearing arrangement comprises at least one, sliding bearing unit intended to carry axial load, and which sliding bearing preferably is water lubricated.
Thanks to said design a cost-efficient solution is obtained which provides for weight reduction and for obtaining high power density. Furthermore, the design may meet heavy demands on operation safety during extreme conditions in certain respects.
According to further aspects of the invention:
said shaft comprises a shaft journal with a flange means showing at least an axial surface intended for the interaction with a sliding bearing;
the flange means is provided with two opposite surfaces interacting with a front and a rear axial sliding bearing, respectively;
there is a front and rear axial sliding bearing and that said front sliding bearing has a considerably larger surface than said rear sliding bearing, wherein preferably the surface of the front bearing is at least 1.5 times as large as the surface of the rear bearing;
said bearing arrangement comprises a radial sliding bearing, which is preferably provided rear of at least one axial bearing unit; and
a conduit system for the supply of a lubricant to said sliding bearing arrangement, wherein preferably at least one of said conduits is provided in a guide vane.
According to another preferred aspect of the invention, the shaft consists of a low weight shaft, which has considerably lower bending rigidity than a conventional steel shaft.
Because of the use of a light weight shaft, which becomes comparatively weak as to bending, conditions are created to use a bearing arrangement which is rigid with reference to bending moments and which handles an axial load and at the same time has non-flexible couplings (e.g. fixedly attached by screws) between the impeller and the end portion of the driving shaft. At the same time, the comparatively weak driving shaft meet the objective to achieve a weight reduction. Further, it makes a cost saving possible with reference to the shaft as the choice of material is optimised in this respect. The shaft may thus be made comparatively slender, and because of the preferred attachment directly against the impeller, optimal conditions are obtained to create as good flow paths as possible, which in turn may imply reduced bending forces influencing the bearing arrangement of the impeller.
According to a preferred embodiment of such a driving system, the driving shaft consists at least mainly of a composite material. Above all, a composite shaft has the great advantage that very low weights may be obtained. A weight reduction of up to 70% as compared to a conventional steel shaft is possible. Further, the advantage is obtained that a composite shaft is exceptionally bendable, which is an advantage with reference to the bearing arrangement. A low bending rigidity is also desirable and a composite shaft may give a reduction of the bending rigidity of about 80% as compared to a conventional, homogenous steel shaft.
According to further potential aspects:
said light weight shaft is made of metal, preferably titanium and/or a hollow steel shaft;
the driving force is transmitted by at least one non-flexible coupling to the stator shell;
the inlet diameter D of said impeller housing is between 0,5-2 m and that the power density is at least 0,5+(2xe2x88x92D) kW/kg,
there is no flexible coupling for the transmission of power from the shaft to the impeller.
Thanks to the invention, it is possible, as compared to conventional systems, to build a substantially much lighter driving system for a water-jet driven ship, which system at the same time provides for a high reliability in operation.